refactor(frontend): remove redundant 'Sync All' button from MCP panel

All MCP operations already auto-sync to live configs:
- upsert_server() → sync_server_to_apps()
- toggle_app() → sync_server_to_app() or remove_server_from_app()
- delete_server() → remove_server_from_all_apps()

The manual 'Sync All' button was redundant and could confuse users
into thinking they need to manually sync after each change.

Changes:
- Remove 'Sync All' button from UnifiedMcpPanel header
- Remove useSyncAllMcpServers hook
- Remove handleSyncAll function and syncAllMutation state
- Remove RefreshCw icon import
- Remove sync-related i18n translations (en/zh)

Note: Backend sync_all_mcp_servers command remains for potential
future use (e.g., recovery tool), but is no longer exposed in UI.

src/hooks/useMcp.ts

@@ -59,12 +59,3 @@ export function useDeleteMcpServer() {
     },
   });
 }
-
-/**
- * 同步所有启用的 MCP 服务器到各应用的 live 配置
- */
-export function useSyncAllMcpServers() {
-  return useMutation({
-    mutationFn: () => mcpApi.syncAllServers(),
-  });
-}